台湾中华大学-数字通信系统与WLAN课程-OFDM.pptVIP

Orthogonal Frequency Division Multiplexing Tung-Chou Chen January 25, 2006 Outlines Introduction to OFDM Wireless Channel Discrete Fourier Transform FFT OFDM Principle Applications of OFDM Introduction to OFDM Introduction to OFDM Introduction Because of its high-speed data transmission and effectiveness in combating the frequency selective fading channel, OFDM technique is widely used in wireless communication nowadays. The basic principle of OFDM is to split a high-rate data stream into a number of lower rate stream that are transmitted simultaneously over a number of subcarriers. OFDM is a multi-carrier transmission technique, which divides the available spectrum into many subcarriers, each one being modulated by a low data rate stream. Introduction to OFDM Introduction The employment of Discrete Fourier Transform to replace the banks of sinusoidal generator and the demodulation significantly reduced the implementation complexity of OFDM modems. Inter-symbol interference (ISI) is eliminated almost completely by introducing a guard time in every OFDM symbol. In the guard time , the OFDM symbol is cyclically extended to avoid inter-carrier interference (ICI). OFDM System Architecture FFT-Based OFDM Systems OFDM and Multicarrier Transmission Modulation OFDM and Multicarrier Transmission Elimination of ISI OFDM and Multicarrier Transmission Bandwidth Usage Advantages and Disadvantages of OFDM The advantages of OFDM Immunity to delay spread and multipath Resistance to frequency selective fading Simple equalization Efficient bandwidth usage The disadvantages of OFDM Synchronization Need FFT units at transmitter and receiver Sensitive to carrier frequency offset High peak to average power ratio Wireless Channel Wireless Transmission Wireless Transmission Large-Scale Small-Scale Fading Large-Scale Small-Scale Fading Mathematical Representation Transmitted Signal s(t) = Re{g(t)exp(j2pfct)} Baseband waveform: g(t) = |g(t)|ejf(t) = R(t)ejf(t) Received Signa